Notice the callback is called with 3 parameters.
First the error argument.
Next the rows returned by the query.
Finally the full result object which contains the same reference to the rows at result.rows

query.first does a bit of argument type checking. If you pass an array as the values argument it is passed to the query unchanged. If you pass anything else truthy it is wrapped inside an array. This makes query.first easier to compose and use.

pg-query uses a random pooled database client for each query.
If you're using a transaction (eg BEGIN/COMMIT) you need to check out a client from the pool manually.
Repeat: DO NOT USE THIS FOR RUNNING TRANSACTIONS